These investigations were conducted to further define the protective extracellular antioxidative mechanisms of various cell types. Proliferating and metabolically active cells, when incubated in the presence of disulfides, thiols of antioxidants, develop the capacity to cleave extracellular disulfides and show enhanced cell membrane-associate Gamma-glutamyl transpeptidase (Gamma-GTP) activity. The thiols released into the medium by these two mechanisms prevent oxidative damage to the cells, a condition permissive for enhanced cell function and proliferation. In all work on this project, cell proliferation, disulfide cleaving activity and Gamma-GTP activity increased proportionally under conditions of cell stimulation and decreased proportionally under conditions of cell inhibition. Even the specific inhibitor of the Gamma-GTP enzyme, Acivicin, led to a proportional diminution of proliferation and disulfide cleaving activity also. Inhibition of glutathione synthesis by buthionine sulfoximine had no effect on development of Gamma-GTP activity, but inhibited proliferation and disulfide cleaving activity. Furthermore, the inhibition was preventable by the inclusion of 2-mercaptoethanol in the incubation medium. These data suggest the dependence of proliferation and disulfide cleaving activity on synthesis of endogenous glutathione, whereas development of Gamma-GTP activity can utilize extracellular glutathione. The agents which induce both disulfide cleaving and Gamma-GTP activities include certain simple thiols and disulfides, non-thiol antioxidants and electron transfer reagents. These data suggest that any change in the redox potential of the culture medium signals the cell to initiate these antioxidant activities. Further studies of the control mechanisms for development of disulfide cleaving and Gamma-GTP activities are being conducted using transformed cells lines. Selection of cell lines enriched or diminished for one or both activities is in progress so as to permit better definition of induction and inhibition of each activity.